第4.5篇-常量池解析（2）

在parse_constant_pool()方法中调用parse_constant_pool_entries()方法对常量池中的各个项进行解析，方法的实现如下：

void ClassFileParser::parse_constant_pool_entries(int length, TRAPS) {
  // Use a local copy of ClassFileStream. It helps the C++ compiler to optimize
  // this function (_current can be allocated in a register, with scalar
  // replacement of aggregates). The _current pointer is copied back to
  // stream() when this function returns. DON'T call another method within
  // this method that uses stream().
  ClassFileStream*  cfs0 = stream();
  ClassFileStream   cfs1 = *cfs0;
  ClassFileStream*  cfs  = &cfs1;

  Handle class_loader(THREAD, _loader_data->class_loader());

  // Used for batching symbol allocations.
  const char*   names[SymbolTable::symbol_alloc_batch_size];
  int           lengths[SymbolTable::symbol_alloc_batch_size];
  int           indices[SymbolTable::symbol_alloc_batch_size];
  unsigned int  hashValues[SymbolTable::symbol_alloc_batch_size];
  int           names_count = 0;

  // parsing  Index 0 is unused
  for (int index = 1; index < length; index++) {
    // Each of the following case guarantees one more byte in the stream
    // for the following tag or the access_flags following constant pool,
    // so we don't need bounds-check for reading tag.
    u1 tag = cfs->get_u1_fast();
    switch (tag) {
      case JVM_CONSTANT_Class :
        {
          cfs->guarantee_more(3, CHECK);  // name_index, tag/access_flags
          u2 name_index = cfs->get_u2_fast();
          _cp->klass_index_at_put(index, name_index);
        }
        break;
      case JVM_CONSTANT_Fieldref :
        {
          cfs->guarantee_more(5, CHECK);  // class_index, name_and_type_index, tag/access_flags
          u2 class_index = cfs->get_u2_fast();
          u2 name_and_type_index = cfs->get_u2_fast();
          _cp->field_at_put(index, class_index, name_and_type_index);
        }
        break;
      case JVM_CONSTANT_Methodref :
        {
          cfs->guarantee_more(5, CHECK);  // class_index, name_and_type_index, tag/access_flags
          u2 class_index = cfs->get_u2_fast();
          u2 name_and_type_index = cfs->get_u2_fast();
          _cp->method_at_put(index, class_index, name_and_type_index);
        }
        break;
      case JVM_CONSTANT_InterfaceMethodref :
        {
          cfs->guarantee_more(5, CHECK);  // class_index, name_and_type_index, tag/access_flags
          u2 class_index = cfs->get_u2_fast();
          u2 name_and_type_index = cfs->get_u2_fast();
          _cp->interface_method_at_put(index, class_index, name_and_type_index);
        }
        break;
      case JVM_CONSTANT_String :
        {
          cfs->guarantee_more(3, CHECK);  // string_index, tag/access_flags
          u2 string_index = cfs->get_u2_fast();
          _cp->string_index_at_put(index, string_index);
        }
        break;
      case JVM_CONSTANT_MethodHandle :
      case JVM_CONSTANT_MethodType :
        if (tag == JVM_CONSTANT_MethodHandle) {
          cfs->guarantee_more(4, CHECK);  // ref_kind, method_index, tag/access_flags
          u1 ref_kind = cfs->get_u1_fast();
          u2 method_index = cfs->get_u2_fast();
          _cp->method_handle_index_at_put(index, ref_kind, method_index);
        } else if (tag == JVM_CONSTANT_MethodType) {
          cfs->guarantee_more(3, CHECK);  // signature_index, tag/access_flags
          u2 signature_index = cfs->get_u2_fast();
          _cp->method_type_index_at_put(index, signature_index);
        } else {
          ShouldNotReachHere();
        }
        break;
      case JVM_CONSTANT_InvokeDynamic :
        {
          cfs->guarantee_more(5, CHECK);  // bsm_index, nt, tag/access_flags
          u2 bootstrap_specifier_index = cfs->get_u2_fast();
          u2 name_and_type_index = cfs->get_u2_fast();
          if (_max_bootstrap_specifier_index < (int) bootstrap_specifier_index)
            _max_bootstrap_specifier_index = (int) bootstrap_specifier_index;  // collect for later
          _cp->invoke_dynamic_at_put(index, bootstrap_specifier_index, name_and_type_index);
        }
        break;
      case JVM_CONSTANT_Integer :
        {
          cfs->guarantee_more(5, CHECK);  // bytes, tag/access_flags
          u4 bytes = cfs->get_u4_fast();
          _cp->int_at_put(index, (jint) bytes);
        }
        break;
      case JVM_CONSTANT_Float :
        {
          cfs->guarantee_more(5, CHECK);  // bytes, tag/access_flags
          u4 bytes = cfs->get_u4_fast();
          _cp->float_at_put(index, *(jfloat*)&bytes);
        }
        break;
      case JVM_CONSTANT_Long :
        {
          cfs->guarantee_more(9, CHECK);  // bytes, tag/access_flags
          u8 bytes = cfs->get_u8_fast();
          _cp->long_at_put(index, bytes);
        }
        index++;   // Skip entry following eigth-byte constant, see JVM book p. 98
        break;
      case JVM_CONSTANT_Double :
        {
          cfs->guarantee_more(9, CHECK);  // bytes, tag/access_flags
          u8 bytes = cfs->get_u8_fast();
          _cp->double_at_put(index, *(jdouble*)&bytes);
        }
        index++;   // Skip entry following eigth-byte constant, see JVM book p. 98
        break;
      case JVM_CONSTANT_NameAndType :
        {
          cfs->guarantee_more(5, CHECK);  // name_index, signature_index, tag/access_flags
          u2 name_index = cfs->get_u2_fast();
          u2 signature_index = cfs->get_u2_fast();
          _cp->name_and_type_at_put(index, name_index, signature_index);
        }
        break;
      case JVM_CONSTANT_Utf8 :
        {
          cfs->guarantee_more(2, CHECK);  // utf8_length
          u2  utf8_length = cfs->get_u2_fast();
          u1* utf8_buffer = cfs->get_u1_buffer();
          assert(utf8_buffer != NULL, "null utf8 buffer");
          // Got utf8 string, guarantee utf8_length+1 bytes, set stream position forward.
          cfs->guarantee_more(utf8_length+1, CHECK);  // utf8 string, tag/access_flags
          cfs->skip_u1_fast(utf8_length);

          if (EnableInvokeDynamic && has_cp_patch_at(index)) {
            Handle patch = clear_cp_patch_at(index);

            char* str = java_lang_String::as_utf8_string(patch());
            // (could use java_lang_String::as_symbol instead, but might as well batch them)
            utf8_buffer = (u1*) str;
            utf8_length = (int) strlen(str);
          }

          unsigned int hash;
          Symbol* result = SymbolTable::lookup_only((char*)utf8_buffer, utf8_length, hash);
          if (result == NULL) {
            names[names_count] = (char*)utf8_buffer;
            lengths[names_count] = utf8_length;
            indices[names_count] = index;
            hashValues[names_count++] = hash;
            if (names_count == SymbolTable::symbol_alloc_batch_size) {
              SymbolTable::new_symbols(_loader_data, _cp, names_count, names, lengths, indices, hashValues, CHECK);
              names_count = 0;
            }
          } else {
            _cp->symbol_at_put(index, result);
          }
        }
        break;
      default:
        classfile_parse_error("Unknown constant tag %u in class file %s", tag, CHECK);
        break;
    }
  }

  // Allocate the remaining symbols
  if (names_count > 0) {
    SymbolTable::new_symbols(_loader_data, _cp, names_count, names, lengths, indices, hashValues, CHECK);
  }

  cfs0->set_current(cfs1.current());
}

循环处理length个常量池项，不过第一个常量池项不需要处理，所以循环下标index的值初始化为1。

如果要了解各个常量池项的具体结构，代码的逻辑理解起来其实并不难。所有项的第一个字节都是用来描述常量池元素类型，调用cfs->get_u1_fast()获取元素类型后，就可以通过switch语句分情况进行处理。

1、JVM_CONSTANT_Class项的解析

JVM_CONSTANT_Class格式如下：

CONSTANT_Class_info {
    u1 tag;
    u2 name_index;
}

调用cfs->get_u2_fast()方法获取name_index，然后调用_cp->klass_index_at_put()方法进行存储。_cp的类型为ConstantPool*，ConstantPool类中的klass_index_at_put()方法的实现如下：

// For temporary use while constructing constant pool
void klass_index_at_put(int which, int name_index) {
    tag_at_put(which, JVM_CONSTANT_ClassIndex);
    *int_at_addr(which) = name_index;
}

void tag_at_put(int which, jbyte t) {
    tags()->at_put(which, t);
}

jint* int_at_addr(int which) const {
     assert(is_within_bounds(which), "index out of bounds");
     return (jint*) &base()[which];
}
intptr_t* base() const {
	  return (intptr_t*) (
			        (  (char*) this  ) + sizeof(ConstantPool)
			  );
}

常量池项的下标与数组的下标是相同的，也就是说，如果当前JVM_CONSTANT_Class存储在常量池中的下标为1处，则也要存储到tags数组中下标为1的地方。同时要将名称索引name_index保存到ConstantPool中存储数据区的对应位置上。

举个例子如下：

#1 = Class         #5        // TestClass
...
#5 = Utf8          TestClass　

假设JVM_CONSTANT_Class是常量池第一项，则解析完这一顶后的ConstantPool对象如下图所示。

 

其中#0（表示常量池索引0）的值为0是因为在分配内存时会将其内存清零。 

2、CONSTANT_Fieldref_info项的解析

格式如下：

CONSTANT_Fieldref_info {
	u1 tag;
	u2 class_index;
	u2 name_and_type_index;
}

调用field_at_put()存储class_index与name_and_type_index，方法的实现如下：

void field_at_put(int which, int class_index, int name_and_type_index) {
    tag_at_put(which, JVM_CONSTANT_Fieldref);
    *int_at_addr(which) = ((jint) name_and_type_index<<16) | class_index;
}

name_and_type_index存储在高16位，class_index存储在低16位。　　

3、JVM_CONSTANT_Methodref项的解析

JVM_CONSTANT_Methodref项的格式如下：

CONSTANT_Methodref_info {
    u1 tag;
    u2 class_index;
    u2 name_and_type_index;
}

按照格式读取Class文件，获取到相关属性值后调用ConstantPool的method_at_put()方法进行存储，这个方法的实现如下：

void method_at_put(int which, int class_index, int name_and_type_index) {
    tag_at_put(which, JVM_CONSTANT_Methodref);
    *int_at_addr(which) = ((jint) name_and_type_index<<16) | class_index;
}

由于ConstantPool数据区一个槽是一个指针类型的宽度，所以至少有32个位，又由于class_index与name_and_type_index属性的类型为u2，这时候就可以使用高16位存储name_and_type_index，低16位存储class_index即可。　　

4、JVM_CONSTANT_InterfaceMethodref项的解析

格式如下：

CONSTANT_InterfaceMethodref_info {
    u1 tag;
    u2 class_index;
    u2 name_and_type_index;
}

调用的interface_method_at_put()方法的实现如下：

void interface_method_at_put(int which, int class_index, int name_and_type_index) {
    tag_at_put(which, JVM_CONSTANT_InterfaceMethodref);
    *int_at_addr(which) = ((jint) name_and_type_index<<16) | class_index;  // Not so nice
}

5、JVM_CONSTANT_String项的解析　　

格式如下：

CONSTANT_String_info {
    u1 tag;
    u2 string_index;
}

调用的string_index_at_put()方法的实现如下：

void string_index_at_put(int which, int string_index) {
    tag_at_put(which, JVM_CONSTANT_StringIndex);
    *int_at_addr(which) = string_index;
}

6、JVM_CONSTANT_MethodHandle项的解析　　

格式如下：

CONSTANT_MethodHandle_info {
    u1 tag;
    u1 reference_kind;
    u2 reference_index;
}

调用的method_handle_index_at_put()方法的实现如下：

void method_handle_index_at_put(int which, int ref_kind, int ref_index) {
    tag_at_put(which, JVM_CONSTANT_MethodHandle);
    *int_at_addr(which) = ((jint) ref_index<<16) | ref_kind;
}

7、JVM_CONSTANT_MethodType项的解析　　

格式如下：

CONSTANT_MethodType_info {
    u1 tag;
    u2 descriptor_index;
}

调用的method_type_index_at_put()方法的实现如下：

void method_type_index_at_put(int which, int ref_index) {
    tag_at_put(which, JVM_CONSTANT_MethodType);
    *int_at_addr(which) = ref_index;
}

8、JVM_CONSTANT_InvokeDynamic项的解析　　

格式如下：

CONSTANT_InvokeDynamic_info {
    u1 tag;
    u2 bootstrap_method_attr_index;
    u2 name_and_type_index;
}

调用的invoke_dynamic_at_put()方法的实现如下：　　

void invoke_dynamic_at_put(int which, int bootstrap_specifier_index, int name_and_type_index) {
    tag_at_put(which, JVM_CONSTANT_InvokeDynamic);
    *int_at_addr(which) = ((jint) name_and_type_index<<16) | bootstrap_specifier_index;
}

9、JVM_CONSTANT_Integer、JVM_CONSTANT_Float项的解析　　

格式如下：

CONSTANT_Integer_info {
    u1 tag;
    u4 bytes;
}

CONSTANT_Float_info {
    u1 tag;
    u4 bytes;
}

调用的方法分别为int_at_put()和float_at_put()方法，实现如下：

void int_at_put(int which, jint i) {
    tag_at_put(which, JVM_CONSTANT_Integer);
    *int_at_addr(which) = i;
}

void float_at_put(int which, jfloat f) {
    tag_at_put(which, JVM_CONSTANT_Float);
    *float_at_addr(which) = f;
}

10、JVM_CONSTANT_Long、JVM_CONSTANT_Double项的解析　　

格式如下： 

CONSTANT_Long_info {
    u1 tag;
    u4 high_bytes;
    u4 low_bytes;
}

CONSTANT_Double_info {
    u1 tag;
    u4 high_bytes;
    u4 low_bytes;
}

调用的long_at_put()和double_at_put()方法的实现如下：

void long_at_put(int which, jlong l) {
      tag_at_put(which, JVM_CONSTANT_Long);
      // *long_at_addr(which) = l;
      Bytes::put_native_u8((address)long_at_addr(which), *(  (u8*) &l  ));
}

void double_at_put(int which, jdouble d) {
    tag_at_put(which, JVM_CONSTANT_Double);
    // *double_at_addr(which) = d;
    // u8 temp = *(u8*) &d;
    Bytes::put_native_u8((address) double_at_addr(which), *((u8*) &d));
}

调用的Bytes::put_native_u8()方法的实现如下：

static inline void put_native_u8(address p, u8 x)   { *(u8*)p = x; }

11、JVM_CONSTANT_NameAndType项的解析　　

格式如下：

CONSTANT_NameAndType_info {
    u1 tag;
    u2 name_index;
    u2 descriptor_index;
}

调用的name_and_type_at_put()方法的实现如下：

void name_and_type_at_put(int which, int name_index, int signature_index) {
    tag_at_put(which, JVM_CONSTANT_NameAndType);
    *int_at_addr(which) = ((jint) signature_index<<16) | name_index;  // Not so nice
}

12、JVM_CONSTANT_Utf8项的解析　　　　

格式如下：

CONSTANT_Utf8_info {
    u1 tag;
    u2 length;
    u1 bytes[length];
}

在HotSpot虚拟机中，字符串通常都会表示为Symbol对象，这样有利于使用符号表来存储字符串，对于2个相同的字符串来说，完全可以使用同一个Symbol对象来表示。这样就可以在ConstantPool数据区相应槽位上存储指向Symbol的指针即可。

调用SymbolTable::lookup_only()方法从符号表中查找对应的Symbol对象，如果查找不到需要暂时将相关的信息存储到临时的names、lengths、indices与hashValues数组中，这样就可以调用SymbolTable::new_symbols()进行批量添加Symbol对象来提高效率；如果找到对应的Symbol对象，则调用symbol_at_put()方法，实现如下：

void symbol_at_put(int which, Symbol* s) {
    assert(s->refcount() != 0, "should have nonzero refcount");
    tag_at_put(which, JVM_CONSTANT_Utf8);
    *symbol_at_addr(which) = s;
}

Symbol** symbol_at_addr(int which) const {
    assert(is_within_bounds(which), "index out of bounds");
    return (Symbol**) &base()[which];
}

将指向Symbol对象的指针存储到指定的位置。　

如果Symbol对象表示的是类名称，那么后面是类连接后，相应索引位置上的值会更新为指向InstanceKlass实例的指针，后面会详细介绍。

 

 

　 　 

参考：

（1）https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-4.html#jvms-4.4

（2）《Java虚拟机原理图解》 1.2.2、Class文件中的常量池详解（上）